This invention relates to a process of improving the melt processing of synthetic polyamides.
Synthetic polyamides, often referred to as Nylons, are widely used engineering plastics due to their excellent mechanical properties at high temperatures, high strength, stiffness, wear and abrasion resistance, low friction and good chemical resistance. Nylons of all types are used in many industries. The most common polyamide products are fibers and injection molded articles. In the course of the preparation of final product, polyamides have to be exposed to melt processing operations in various types of extruders.
Extrusion operations are complex processing operations by which the raw material formulations are converted into a continuous and often complex extrudate. The extruder geometry and the operating conditions are extremely important parameters that can influence the melting of the polymer and hence also output rate of the extrusion process and uniformity of the extrudate, which is of crucial importance for stability of the process as well as for properties of the final polymer product. With the known stabilizers it is not possible to influence the melt processing properties of synthetic polyamides.
It is the object of the present invention to provide a unique stabilizer specially tailored for use in polyamides to overcome the above-mentioned short-comings.
In WO 97/43335 a process of increasing the heat stability, light stability, chemical stability and dye affinity of synthetic polyamides is described which consists in the addition of an additive having the formula (I) as defined below and it has been disclosed that the addition of such additive also improves the spinnability of fibers consisting of synthetic polyamides. The cited document is however silent on the improvement of processability in non-fibers applications and of fibers which have been colored in the mass.
It has now been found that the addition of an additive having the formula (I) as defined below increases quite generally the stability of polyamide melt pressure which leads to higher throughput and quality of injection molded or extruded polyamide articles. This effect is surprising because it was not observed until now with other derivatives of 2,2,6,6-tetra-methylpiperidine.
In a study on polyamide extrusion parameters it was found that the uniformity of the extrusion process and the output of the extruder can be significantly influenced by the addition of an additive having the formula (I) as defined below.
The invention, therefore, provides in one of its aspects a method of improving the melt processing of synthetic polyamides which have been colored in the mass and in non-fiber applications of synthetic polyamides and in particular pigmented and/or filled synthetic poly-amides by mixing therewith an effective amount of an additive according to the formula (I) 
wherein R1 represents a methyl group.
By adding an effective amount of the additive hereinabove described to synthetic polyamide one is able to improve the melt processing stability of the dyed or pigmented polyamide without affecting certain desirable physical characteristics of the polymer, for example the relative viscosity and the degree of polymerization.
The additive according to formula (I) may be present in the synthetic polyamide in amounts of from 0.01 to 5.0%, preferably 0.05 to 1.0% by weight based on the total weight of the stabilized synthetic polyamide.
The polyamide to be stabilized can be a homopolyamide, a copolyamide, a mixture or blend of polyamides or of a polyamide and another polymer.
Preference is given to homopolyamides and/or copolyamides made from xcfx89-aminocaproic acid, xcfx89-aminoenanthoic acid, xcfx89-aminocaprylic acid, xcfx89-aminopelargonic acid, xcfx89-aminocapric acid, xcfx89-aminoundecanoio acid, a)-aminolauric acid, caprolactam, lactam-7, lactam-8, lactam-9, lactam-10, lactam-11 and/or laurolactam.
The polyamides to be stabilized can also be selected from the group of polyamides made, for example from dimethylenediamine, trimethylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, polyetherdiamine and mixtures thereof on the one hand, and oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, nonedicarboxylic acid, decanedicarboxylic acid, undecanedioic acid, dodecanedioic acid, dimerised fatty acids and mixtures thereof on the other hand.
The preparation of synthetic polyamides stabilized according to the invention can be effected by conventional means, preferably by melt blending of the additive of formula (I) with synthetic polyamide in a suitable equipment, for example in an extruder prior to molding or spinning operations. The additive can also be incorporated prior or during the polycondensation process in preparation of the synthetic polyamide itself.
Synthetic polyamides used in the invention are preferably dyed or pigmented, for example by topical application or by application in the melt in accordance with conventional methodology. Any type of dyestuff or pigment is suitable provided that it is stable at the high temperatures encountered in the melt. Preferred dyestuffs are monoazo complexes, in particular, the chromium complexes that are sufficiently stable at the high working temperatures encountered in, e.g. a spinning process. Preferred reactive dyestuffs are those halogen-containing triazinyl or vinyl group-containing metallized azo dyestuffs, in particular, those metallized with chromium, nickel or copper. Preferred pigments are, for example Carbon black, Pigment Yellow 192, Pigment Orange 68 and Pigment Red 149. Preferred polymer soluble dyes are for example Solvent Red 179.
If dyestuffs or pigments are employed in the synthetic polyamides they are preferably employed in amounts of from 0.1 to 10 parts, more preferably, 0.1 to 4 parts per 100 parts of the synthetic polyamide.
Synthetic polyamides may additionally comprise fillers or fibers, for example glass spheres or glass fibers, and/or delusterants, for example titanium dioxide which may be present in proportions of from 0.1 to 5.0 parts, more preferably 0.2 to 2.0 parts, e.g. 0.2 parts per 100 parts of polyamide.
The invention provides in a further one of its aspects a modified synthetic polyamide obtainable by the process hereinabove defined which polyamide has improved melt processing stability.